#define _CRT_SECURE_NO_WARNINGS 1
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    long long prev = -1e12;
    bool isValidBST(TreeNode* root) {
        if (root == nullptr) return true;

        bool left = isValidBST(root->left);
        if (left == false) return false;
        bool cur;
        if (left == true && prev < root->val) prev = root->val, cur = true;
        else cur = false;
        bool right = isValidBST(root->right);
        return left && cur && right;
    }
};